Mass spectrometers are an apparatus for ionizing a variety of substances contained in a sample and separating the ions in every mass (specifically, the mass-to-charge ratio m/z) to detect them. With a mass spectrometer, the mass of each ion species can be accurately obtained. In identifying the substance contained in the sample, a database may be previously created in which the masses of a variety of substances whose compositions and structures are known are registered. Accordingly, a substance can be identified by comparing the masses of the peaks appearing on the mass spectrum obtained by a mass analysis and the database. Even for a substance which is not registered in the database, if the kinds of elements and the numbers of atoms of these elements constituting the substance are known to some extent, the composition of the unknown substance can be deduced by determining the consistency between the mass of the unknown substance to be identified and the mass obtained by combining a variety of elements.
However, even in the case where a certain unknown substance is identified by a search using a database as previously described for example, the reliability is not necessarily high. That is, a certain level of tolerance is normally set for a mass, in consideration of the mass error of the measurement and mass difference of the isotopes, and the substance is considered to be the correct substance if the consistency is shown within the mass range. However, even a substance whose composition formula is not registered in the database often falls into the mass range. Therefore, although in reality such a substance is the correct substance, in some cases an erroneous identification may be performed.
On the other hand, with an MS/MS (or MSn) mass spectrometer, an ion having a specific mass can be selectively dissociated, and a variety of product ions (or fragment ions) generated by this process are mass analyzed to create an MSn mass spectrum. In this case, a database is previously created in which the mass and intensity (or fragment pattern) of the product ions are registered for a large number of compositions. Then, the compound can be identified by comparing the mass and intensity of each peak appearing on the MSn mass spectrum and the database (refer to Patent Document 1 for example).
However, even such an identification method does not always provide a correct result, since a dissociation has more than one mode and it is impractical to register all the fragment patterns in the database.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. H08-124519 (paragraphs [0002] through [0004])